See also

Verb

French

Noun

Chemical synapses, are specialized junctions
through which neurons
signal to each other and to non-neuronal cells such as those in
muscles or glands. Chemical synapses allow
neurons to form interconnected circuits within the central
nervous system. They are thus crucial to the biological
computations that underlie perception and thought. They provide the
means through which the nervous system connects to and controls the
other systems of the body, for example the specialized synapse
between a motor neuron and a muscle cell is called a neuromuscular
junction.

Young children have about 1016 synapses (10
quadrillion).
This number declines with age, stabilizing by adulthood. Estimates
for adults vary from 1015 to 5 × 1015 (1-5 quadrillion)
synapses.

The word "synapse" comes from "synaptein", which
Sir Charles
Scott Sherrington and colleagues coined from the Greek "syn-"
("together") and "haptein" ("to clasp"). Chemical synapses are not
the only type of biological synapse: electrical
and immunological
synapses exist as well. Without a qualifier, however, "synapse"
commonly refers to a chemical synapse.

Structure

Chemical synapses pass information directionally
from a presynaptic cell to a postsynaptic cell and are therefore
asymmetric in structure and function. The presynaptic terminal, or
synaptic bouton, is a specialized area within the axon of the presynaptic cell that
contains neurotransmitters
enclosed in small membrane bound spheres called synaptic
vesicles. Synaptic vesicles are docked at the presynaptic
plasma
membrane at regions called active zones (AZ).

Immediately opposite is a region of the
postsynaptic cell containing neurotransmitter receptors; for
synapses between two neurons the post synaptic region may be found
on the dendrites or
cell body. Immediately behind the postsynaptic membrane is an
elaborate complex of interlinked proteins called the postsynaptic
density (PSD).

Proteins in the PSD are involved in anchoring and
trafficking neurotransmitter receptors and modulating the activity
of these receptors. The receptors and PSDs are often found in
specialized protrusions from the main dendritic shaft called
dendritic
spines.

Between the pre and postsynaptic cells is a gap
about 20nm wide called the synaptic
cleft. The small volume of the cleft allows neurotransmitter
concentration to be raised and lowered rapidly. The membranes of
the two adjacent cells are held together by cell
adhesion proteins.

Signaling across chemical synapses

Neurotransmitter release

The release of a neurotransmitter
is triggered by the arrival of a nerve impulse (or action
potential) and occurs through an unusually rapid process of
cellular secretion, also known as exocytosis: Within the
presynaptic nerve terminal, vesicles
containing neurotransmitter sit "docked" and ready at the synaptic
membrane. The arriving action potential produces an influx of
calcium
ions through
voltage-dependent, calcium-selective ion channels at the down
stroke of the action potential (tail current). Calcium ions then
trigger a biochemical cascade which results in vesicles fusing with
the presynaptic membrane and releasing their contents to the
synaptic cleft within 180µsec of calcium entry. This
effect is utilized with clonidine to perform
inhibitory effects on the SNS.

Heterotropic modulation

Heterotropic modulation is a
modulation of presynaptic terminals of nearby neurons. Again, the
modulation can include size, number and replenishment rate of
vesicles.

One example are again neurons of the sympathetic
nervous system, which release noradrenaline, which, in
addition, generate inhibitory effect on presynaptic terminals of
neurons of the
parasympathetic nervous system. On the other hand, a
presynaptic neuron releasing an inhibitory neurotransmitter such as
GABA can cause
inhibitory postsynaptic potential in the post-synaptic neuron,
decreasing its excitability and therefore decreasing the neuron's
likelihood of firing an action potential. In this way, the output
of a neuron may depend on the input of many others, each of which
may have a different degree of influence, depending on the strength
of its synapse with that neuron. John
Carew Eccles performed some of the important early experiments
on synaptic integration, for which he received the
Nobel Prize for Physiology or Medicine in 1963. Complex
input/output relationships form the basis of transistor-based computations
in computers, and are
thought to figure similarly in neural circuits.

Synaptic strength

The strength of a synapse is defined by
the change in transmembrane potential resulting from activation of
the postsynaptic neurotransmitter receptors. This change in voltage
is known as a postsynaptic potential, and is a direct result of
ionic currents
flowing through the postsynaptic ion channels. Changes in synaptic
strength can be short–term and without permanent
structural changes in the neurons themselves, lasting seconds to
minutes — or long-term (long-term
potentiation, or LTP), in which repeated or continuous synaptic
activation can result in second
messenger molecules initiating protein
synthesis, resulting in alteration of the structure of the
synapse itself. Learning and memory are believed to result from
long-term changes in synaptic strength, via a mechanism known as
synaptic
plasticity.

Relationship to electrical synapses

An electrical
synapse is a mechanical and electrically conductive
link between two abutting neurons that is formed at a
narrow gap between the pre- and postsynaptic cells
known as a gap
junction. At gap junctions, cells approach within about
3.5 nm of each other,
rather than the 20 to 40 nm distance that separates cells
at chemical synapses. As opposed to chemical synapses, the
postsynaptic potential in electrical synapses is not caused by the
opening of ion channels by chemical transmitters, but by direct
electrical coupling between both neurons. Electrical synapses are
therefore faster and more reliable than chemical synapses.
Electrical synapses are found throughout the nervous system, yet
are less common than chemical synapses.